Control of particle morphology and homogeneity during spray pyrolysis: Diffusion drying stage.

摘要

The objective of this study is to develop the capabilities to determine the effect of operational parameters on the morphology of powders produced by spray pyrolysis—particularly the control of hollow or solid particles.; A theoretical model has been constructed to predict the development of particle morphology during drying process. This model is used to describe the evaporation of droplet containing dissolved solid undergoing diffusion drying. Included in the model were the solute crystallization and crust fragmentation mechanisms, which were major innovations in thesis. The model predicts the solute concentration and droplet temperature profiles as a function of time. The first stage of drying is characterized by the solvent evaporation from a free liquid surface. The second stage of drying is characterized by the solvent evaporation through a porous shell. During the second stage of drying, the droplet can be viewed as composed of three regions. They are (1) central wet kernel (unsaturated liquid); (2) porous shell (saturated liquid and solid mixture); and (3) ambient gas–vapor mixture. During this stage, the drop temperature as well as inner partial pressure rises rapidly once the drop forms a solid crust at the surface. Fragmentation occurs when the partial pressure inside the drop overcomes the fragmentation energy of the crust.; A device used for monitoring morphological characteristics of a single suspended droplet was constructed. Several materials with different physical properties such as solubility, thermal conductivity, and latent heat of crystallization were investigated. The measurement of droplet diameter is quantitatively realistic in the early drying stage because the droplet is shrinking more or less uniformly. It has been shown that the shell structure of a drying droplet plays an important role in determining the final dried particle morphology. In addition, the shell structure of a drying droplet varies from material to material, and it increases the difficulty in the modeling prediction. In the cases of sodium chloride and ammonium chloride, the model gave good quantitative prediction for low drying temperatures in the first stage of drying process. Two sets of experiments (Ca(C₂H₃O₂)₂ vs. NaC₂H₃O₂ and K₂CO₃ vs. Na₂CO₃) were demonstrated to examine the effect of material solubility. The effect of latent heat of crystallization was examined by experiment (NH₄Cl vs. NaCl). Generally, numerical simulation gave reasonably good qualitative prediction for most of test materials.; In summary, low drying temperature, slow heating rate, slow gas flow rate, and material with low thermal conductivity and high latent heat of crystallization favor the formation of small solid particle. High porosity and high permeability of shell structure lead to small solid particle formation. High relative humidity, small initial size, solvent with high latent heat of evaporation and material with high solubility lead to small, dense but irregular particle shapes. The high initial solute concentration is favorable for the formation of a large dense solid particle. Conditions conductive to solid and/or hollow particle formation are presented in the conclusions of this work. (Abstract shortened by UMI.)